In general, class D audio amplifiers have the benefit of high power efficiency. However, such amplifiers also have a drawback in terms of electromagnetic interference (EMI), which can interfere with nearby wireless receivers, violate FCC emission limits, introduce noise into various signal paths, or any combination thereof. In a particular example related to audio applications, switching frequencies can range from approximately 200 kHz to 1000 kHz. In class D amplifiers, the resulting carrier and its harmonics due to such switching often overlap with the amplitude modulated (AM) frequency band, which ranges from approximately 520 kHz to 1710 kHz. Thus, the class D amplifier may cause EMI that can interfere with nearby AM receivers unless some “mitigation” techniques are used. Class D amplifiers can also be used in connection with switched power supplies, direct-current to direct-current (DC-DC) converters, data converters, motor controllers, other systems that use pulse-width modulation, or any combination thereof. In each such instance, the class D amplifiers can generate electromagnetic interference (EMI) and can interfere with AM frequency bands and other sensitive frequency bands.
In general, class D amplifiers can use a variety of modulation techniques. One common modulation technique is referred to as BD double sided (BD-D) pulse width modulation (PWM). In general, BD-D PWM includes varying the pulse width of two complementary pulse waves where the centers of the pulses are time-aligned and often centered within a pulse width modulated (PWM) frame. For positive input signals, a pulse width of the input signal (typically referred to as P or B) that drives the high side of a bridged output is increased, while a complementary pulse width of the signal that drives the low side (typically referred to as N or D) of the bridged output is reduced. Unfortunately, such BD modulation results in the common mode carrier frequency being centered inconveniently at the frame rate. In a particular example where the switching frequencies for audio applications overlap with the AM band, the common mode carrier and its harmonics can radiate energy in the AM frequency band, interfering with reception of an AM radio receiver in close proximity or within the same system.
One technique employed in the prior art for mitigating AM radio interference includes adjusting the PWM carrier signal away from the desired radio station frequency. While such adjustments may avoid interfering with a co-resident AM radio receiver, it is not practical for avoiding interference with a non co-resident AM receiver (since the desired radio frequency may not be known) and does not help suppress EMI for emission compliance. Further, a technique employed in the prior art for suppressing EMI for emission compliance includes dithering the frequency of the PWM carrier signal. However, the dithering technique provides modest average EMI suppression, sometimes has minimal suppression of instantaneous peak carrier interference, and can adversely impact the integrity of the baseband signal and limit a maximum modulation index of the signal. Therefore, it is desirable to meaningfully suppress the PWM carrier signal power with little or no compromise in the baseband signal performance.